Silica aerogels are very light and highly porous (about 98% air). They offer large internal surface area, extremely low density (0.08 g/cm3 to 0.10 g/cm3), and low thermal conductivity (0.01 W/m−K at −200° C. to 0.035 W/m−K at 100° C.). These properties render them suitable as thermal and/or acoustics insulators, or as optical devices, electrical devices, and/or energy storing devices.
Silica aerogels, however, are brittle. Moreover, they are generally synthesized in monolithic or granular form, which makes their processing and handling difficult. Volumetric shrinkage also occurs, and the effects become increasingly apparent at elevated temperatures. These severely limit their use in various applications. Even though attempts have been made to dope silica aerogels with materials such as polymer, ceramics, and metals to improve their mechanical properties, any such improvements may be compromised by undesirable increases in density as well as reduction in insulation performance.
State of the art methods to produce aerogel hybrid composites include a sol-gel method. Using the sol-gel method, modifications may be made to the silica backbone structure during gelation of the precursors to obtain a stronger, stiffer, and more flexible aerogel. This improvement, however, may be carried out at the expense of increase in density and increase in thermal conductivity of the aerogel hybrid composite. Moreover, large quantities of solvent for fluid exchange are required prior to drying. Hazardous substances used in manufacturing the composites may also compromise workplace safety and health of personnel involved.
Apart from the above-mentioned, binders have been used to modify aerogels to produce solid composites. For example, tackiness of polymeric resins, such as epoxy and ethylene vinyl acetate (EVA) resins, has been leveraged on in promoting adhesion of the solid composites.
Post-synthesized modification of silica aerogels by binders, however, presents another set of problems. Firstly, infiltration of binder material into pores of aerogels may fill the pores partially or completely, resulting in surface area reduction and density increase of the aerogels. Moreover, when polymeric resins are used as binders for silica aerogels, the resins, being much denser, tend to settle at the bottom during curing, and results in a non-uniform mixture. Many of these binding systems use organic solvents such as acetone and toluene in the mixture, which may cause the resins to infiltrate rapidly into the nano-pores. This may collapse the aerogel structure completely, such as that shown in FIG. 1. These solvents are furthermore hazardous to health, and their volatility may compromise safety of personnel working with these chemicals.
In view of the above, there exists a need for improved methods of preparing composites that overcome or at least alleviate one or more of the above-mentioned problems.